1. Field of the Invention
The present invention generally relates to a transmission device connected to a network, and more particularly to a transmission device used for an optical communication network which employs a digital hierarchy.
An optical communication network has been practically used as means for providing broadband services in which a variety of data on telephone, facsimile, images and so on is integrated. The user/network interface in the optical communication network has been internationally standardized, and is known as a Synchronous Digital Hierarchy (SDH), as defined in the CCITT recommendations G707, G708 and G709, the disclosure of which is hereby incorporated by reference. A network which conforms to the SDH has been practically used as SONET (Synchronous Optical NETwork) in the North America.
2. Description of the Prior Art
First, a description will be briefly given of the SONET. The SONET is described in, for example, William Stallings, "ISDN and Broadband ISDN, Macmillan Publishing Company, 1992, pp. 546-558.
In the SONET, a multiplexed optical carrier (OC) is transmitted. The transmission device converts the optical signal (carrier) into an electric signal and vice versa. The electric signal is called a synchronous transport signal (STS). The basic bit rate of the SONET is 51.84 Mbps. The optical carrier having the above basic bit rate is expressed as OC-1. Generally, an optical carrier or signal is expressed as OC-N where N (optical carrier level N) is an integer, and a corresponding electric signal is expressed as STS-N (synchronous transport carrier level N). For example, the optical carrier OC-12 is an optical carrier or signal having a bit rate of 622.080 Mbps (=12.times.51.84 Mbps). In the SONET, signals having bit rates which are integer multiples of the basic bit rate. The optical carrier OC-12 is obtained by multiplexing 12 STS-1 signals at the byte level to thereby generate an STS-12 signal and by converting the STS-12 signal into an optical signal. Generally, the multiplexing of STS-N signals employs a byte-level interleave process.
It will be noted that the STS-3 in the SONET corresponds to a synchronous transport module STM-1 in the SDH. Similarly, the STS-12 corresponds to the STM-4.
As shown in FIG. 1, the signal STS can be obtained by, for example, sequentially multiplexing digital signals having lower bit rates, such as DS-0 (64 Kbps), DS-1 (1.5 Mbps), DS-2 (6.3 Mbps) and DS-3 (45 Mbps).
FIG. 2 is a block diagram showing the outline of a network of the SONET. Electric signals from terminals 1 and 2 are respectively multiplexed by transmission devices 3 and 7, and resultant multiplexed signals are converted into light signals, which are then sent to transmission paths 8 formed of optical fiber cables. Repeaters 4, 5 and 6 are provided in the transmission paths 8. Particularly, the repeater 5 has a function of terminating the optical signals (the above function is called an add/drop function). As shown in FIG. 2, terms "section", "line" and "path" are defined in the SONET. The section corresponds to an optical transmission part between transmission devices, between repeaters or between a transmission device and a repeater. The line corresponds to an optical transmission part between transmission devices, between repeaters or between a transmission device and a repeater, each having the terminating function. The path indicates the end-to-end optical transmission part.
FIG. 3A is a diagram showing the frame format of the signal STS-1. As shown in FIG. 3A, the signal STS-1 consists of 810 octets, and is transferred every 125 .mu.s. The 810 octets consists of nine rows arranged in a matrix formation, each of the rows consisting of 90 octets. In other words, the signal STS-1 has a 9.times.9 matrix formation. The first three columns (three octets.times.nine rows) forms an overhead in which a variety of control information concerning transmissions. The first three rows of the overhead forms a section overhead, and the remaining six rows forms a line overhead. The control information forming the overheads is also referred to as overhead information.
FIG. 3B is a diagram showing the frame format of the signal STS-3. In the SDH, a new format is not created during the hierarchically multiplexing operation. That is, the signal STS-3 can be formed by simply byte-multiplexing the signals STS-1 including the headers thereof without forming a new header specifically directed to the signal STS-3.
FIG. 4A shows the section overhead and the line overhead, and FIG. 4B shows the path overhead. The bytes forming these overheads are well known, and a description thereof will be omitted here.
Generally, the B-ISDN employs a plurality of different clock signals, and each terminal in the B-ISDN uses any of the clock signals. For example, a composite clock signal is used for the DS-0 signal of the 64 kbps. The composite clock signal will be described with reference to FIGS. 5A through 5D.
The composite clock signal is composed of a BPV pulse signal of 64 kbps and a return-to-zero pulse signal of 8 kbps (hereinafter simply referred to as RZ pulse signal). As shown in FIG. 5A, the RZ pulse is superimposed on the BPV pulse signal every eight pulses. As shown in FIG. 5B, the BPV pulse signal alternately converts code "1" into "+1" and "-1". As shown in FIG. 5C, the RZ pulse signal switches has a section "0" for each code. A non-return-to-zero pulse signal is shown in FIG. 5D for reference.
Another clock signal called SF_BITS is used for the DS-1 signal of 1.544 Mbps. The transmission device which processes the DS-1 signal operates in synchronism with the SF_BITS clock signal.
Yet another clock signal called line clock is used for the signal DS-3 (which corresponds to the signal STS-1) and the signal STS-3.
Conventionally, each terminal in the B-ISDN uses the predetermined clock signal as a reference clock. Recently, an arrangement has been proposed in which the clock to be used can be selected by using a synchronization message. The synchronization message is defined in the SONET standard, and indicates the quality level of the reference clock (the precision of the clock). The synchronization message is sometimes called QL information. The synchronization message is transferred between the terminals by means of the given overhead and an ESF_DS1 data link which will be described later. One of the clock signals having the highest quality indicated in the synchronization message is selected as the reference clock or master clock.
Two frame formats of the signal DS-1 have been defined in the SONET standard, one of which is an SF (Super Frame) format and the other is an ESF (Extended Super Frame) format.
FIG. 6 is a diagram of the SF format. One super frame SF consists of 12 frames. Each of the 12 frames in the SF format is made up of 24 time slots (1-24) and a frame bit F. One time slot consists of eight bits. One frame is equal to 125 .mu.s, and one multiframe is equal to 1.5 ms. The 12 frame bits F included in the multiframe of the SF format form a synchronizing signal.
FIG. 7 is a diagram of the ESF format. One extended super frame ESF consists of 24 frames. Each of the 24 frames in the ESF format is made up of 24 time slots and a frame bit F. One time slot consists of eight bits. One frame is equal to 125 .mu.s, and one multiframe is equal to 3.0 ms. The 24 frame bits F included in the multiframe of the ESF format include six bits used to form the synchronizing signal, four bits for CRC channels, and 12 bits used to form a data link signal.
The data link signal composed of the above 12 bits is used to transfer the synchronization message indicative of the qualities of the clock signals.
FIG. 8 shows the relationship between the qualities of the clock signals and the data link signal. FIG. 8 shows five different clock qualities, and shows the corresponding values of the data link signal and the corresponding values of the S1 byte (FIG. 4A). The synchronization message is allowed to indicate indefinite clock precision and inhabitation of use for the reference clock.
The transmission device which is operating in a clock selection mode selects the reference clock by referring to the synchronization message received by the transmission device. The transmission device which sends the synchronization message selects the clock signal having the highest clock quality and specifies the corresponding clock quality information therein.
As shown in FIG. 8, the lowest precision (quality) supported by the synchronization message is equal to .+-.4.6.times.10.sup.-6. The lowest precision is much higher than the possible highest quality of the composite clock signal or the SF_BITS clock signal. In other words, the synchronization message does not support the composite clock signal and the SF_BITS clock signal. Hence, the transmission device which is operating in the clock selection mode using the synchronization message cannot select, as the reference clock signal, the composite clock signal or the SF_BITS clock signal. In addition, the above transmission device cannot notify the other transmission devices of use of the composite clock signal or the SF_BITS clock signal as the reference clock signal.
If a certain factor occurs which prevents the transmission device from selecting the reference clock signal based on the synchronization message, the transmission device no longer operates. Further, the other transmission devices which are notified of the reference clock signal do not operate. Even if these transmission devices receive the composite clock signal or the SF_BITS clock signal, the devices do not select any of the clock signals as the reference clock. As a result, the services related to the transmission devices become not available.